Tissues: Structure, Function, and Classification in Anatomy & Physiology

Chapter 4: Tissues

Biological Levels of Organization

Tissues represent one of the fundamental levels of biological organization, situated between cells and organs. Understanding tissues is essential for comprehending how the body’s structures and functions are integrated.

• Definition: A tissue is a group of similar cells and their extracellular products that perform a specific function.

• Hierarchy: Cells → Tissues → Organs → Organ Systems → Organism

• Example: Muscle tissue contracts to produce movement, while nervous tissue transmits electrical signals.

Main Tissue Types

The human body contains four primary tissue types, each with distinct structures and functions.

• Epithelial Tissue: Covers body surfaces, lines cavities, and forms glands.

• Connective Tissue: Supports, binds, and protects other tissues and organs.

• Muscle Tissue: Responsible for movement through contraction.

• Nervous Tissue: Initiates and transmits electrical impulses for communication.

Epithelial Tissue

Definition and Locations

Epithelial tissue consists of closely packed cells that form continuous sheets, covering surfaces and lining cavities throughout the body.

• Locations: Skin (epidermis), lining of the digestive tract, respiratory passages, blood vessels, and glands.

Characteristics and Functions

• Cellularity: Composed almost entirely of tightly packed cells with minimal extracellular matrix.

• Polarity: Has an apical surface (exposed to body exterior or cavity) and a basal surface (attached to underlying connective tissue).

• Attachment: The basement membrane anchors epithelium to connective tissue.

• Avascularity: Lacks blood vessels; nutrients diffuse from underlying tissues.

• Regeneration: High capacity for renewal due to frequent cell division.

• Functions: Protection, absorption, filtration, excretion, secretion, and sensory reception.

Specializations of the Apical Surface

• Microvilli: Increase surface area for absorption (e.g., intestinal lining).

• Cilia: Move substances across the surface (e.g., respiratory tract).

Basement Membrane

• Definition: A thin, fibrous layer that attaches epithelium to underlying connective tissue.

• Function: Provides support and regulates exchange of materials.

Intercellular Junctions

• Tight Junctions: Prevent passage of substances between cells.

• Desmosomes: Provide mechanical strength by anchoring cells together.

• Gap Junctions: Allow communication and passage of ions between cells.

Classification of Epithelial Tissue

• By Cell Shape:

  • Squamous: Flat, scale-like cells (e.g., alveoli of lungs).

  • Cuboidal: Cube-shaped cells (e.g., kidney tubules).

  • Columnar: Tall, column-like cells (e.g., lining of intestines).

• By Number of Layers:

  • Simple: Single layer (adapted for absorption, filtration).

  • Stratified: Multiple layers (adapted for protection).

Comparison Table: Epithelial Types

Type	Main Function	Location
Simple Squamous	Diffusion, filtration	Alveoli, blood vessels
Simple Cuboidal	Secretion, absorption	Kidney tubules, glands
Simple Columnar	Absorption, secretion	Digestive tract lining
Stratified Squamous	Protection	Skin, mouth, esophagus
Stratified Cuboidal	Protection	Some ducts (rare)
Stratified Columnar	Protection, secretion	Male urethra (rare)

Glands: Exocrine vs. Endocrine

• Gland: A structure that produces and releases substances (secretions).

• Exocrine Glands: Secrete products into ducts (e.g., sweat, salivary glands).

• Endocrine Glands: Release hormones directly into the bloodstream (e.g., thyroid, pituitary).

Connective Tissue

General Characteristics and Functions

• Most abundant and widely distributed tissue type.

• Functions: Support, protection, insulation, storage, and transport.

• Components: Cells, fibers, and ground substance (matrix).

General Components of Connective Tissue

Component	Description	Function
Cells	Fibroblasts, adipocytes, immune cells	Produce matrix, store energy, defense
Fibers	Collagen, elastic, reticular	Strength, elasticity, support
Ground Substance	Gel-like material	Fills space, supports cells

Types of Connective Tissue

• Areolar Tissue: Loose arrangement of fibers; supports and binds other tissues; found under epithelia.

• Adipose Tissue: Stores fat; insulates and cushions organs; found under skin, around kidneys.

• Reticular Tissue: Network of reticular fibers; supports lymphoid organs.

• Dense Regular Tissue: Parallel collagen fibers; provides tensile strength; found in tendons, ligaments.

• Dense Irregular Tissue: Irregularly arranged fibers; withstands tension; found in dermis of skin.

• Cartilage (Hyaline): Firm, flexible support; found in nose, trachea, ends of long bones.

• Bone: Rigid support, protection, mineral storage; forms skeleton.

• Blood: Fluid tissue; transports gases, nutrients, wastes.

Types of Membranes

Body membranes are sheets of tissue that cover or line body surfaces. They can be classified by location, structure, and function.

Type	Location	Purpose	Composition
Mucous	Lines body cavities open to exterior (e.g., digestive, respiratory tracts)	Secretion, protection	Epithelial + connective
Serous	Lines closed body cavities (e.g., peritoneum, pleura)	Reduces friction	Epithelial + connective
Cutaneous	Skin	Protection	Epithelial + connective
Synovial	Lines joint cavities	Lubrication	Connective only

Muscle Tissue

General Function

• Function: Produces movement by contracting; maintains posture; generates heat.

Types of Muscle Tissue

Type	Control	Striations	Nuclei	Location
Skeletal	Voluntary	Striated	Multinucleate	Attached to bones
Cardiac	Involuntary	Striated	Uninucleate (mostly)	Heart
Smooth	Involuntary	Non-striated	Uninucleate	Walls of hollow organs

Nervous Tissue

General Function

• Function: Detects stimuli, processes information, and coordinates responses via electrical impulses.

Cell Types

• Neurons: Transmit electrical signals; found in brain, spinal cord, nerves.

• Neuroglia (glial cells): Support, protect, and nourish neurons.

Inflammation and Tissue Repair

• Inflammation: A protective response to injury or infection, characterized by redness, heat, swelling, pain, and loss of function.

• Role in Repair: Inflammation isolates damaged area, removes debris, and initiates healing.

Aging and Tissue Structure

• Aging Effects: Tissues may lose elasticity, repair more slowly, and accumulate damage with age.

• Examples: Thinner skin, reduced bone density, decreased muscle mass.

Additional info: Some details, such as rare locations of certain epithelial types and the specific cell types in connective tissue, were inferred for completeness.